Steel cords for the reinforcement of rubber articles and method of producing the same

Abstract: A steel cord for the reinforcement of rubber articles has a three-layer twisting structure comprising a core (1) of 2 steel filaments, a middle sheath layer (2) of 6 steel filaments, and an outer sheath layer (3) of 11 steel filaments, in which a ratio ds/dc of filament diameter ds in the middle and outer sheath layers to filament diameter dc in the core is within a range of 1.15-1.5, and a twisting pitch of the core is not less than 20 mm. Such steel cord may be used as a reinforcing member in for example a heavy duty pneumatic radial tire or a conveyor belt.

Description: This invention relates to steel cords for the reinforcement of rubber articles such as heavy duty pneumatic radial tires, conveyor belts and the like, and to a method of producing such steel cords. In rubber articles including pneumatic tires, steel cords are widely used as a reinforcing member. For example, steel cords having 3+9+15 twisting structure by varying a twisting pitch of steel filaments having the same filament diameter every layer are widely used as steel cord for heavy duty tires in order to provide a high cord breaking load per diameter of the cord and to provide good fatigue resistance. However, the steel cord of such a structure has no space for penetrating rubber into the inside of the cord as shown in Fig. 4 of the accompanying drawings, so that if the cord contacts water, corrosion of the cord is caused by water penetrating into the spaces inside the cord containing no rubber. Furthermore, there is known a steel cord of compact twisting structure in which 24 steel filaments are simultaneously twisted around 2 filaments as a core as shown in Fig. 3 of the accompanying drawings. Such a steel cord also has poor corrosion resistance because there is no space for penetrating rubber in the inside of the cord. In order to solve such a corrosion problem, JP-B-62-18678 and JP-A-4-257382 propose steel cords capable of improving rubber penetrability into the inside of the cord by decreasing the number of steel filaments used in the sheath layer and forming a space between adjoining steel filaments in the sheath layer, respectively. Furthermore, JP-U-64-30398 proposes that the corrosion problem can be solved and the working cost in the twisting step can be reduced by aligning the steel filaments in the core layer of steel cord of two-layer or three-layer twisting structure in parallel with each other. The steel cord proposed in JP-B-62-18678 has a three-layer twisting structure consisting of a core formed by twisting 2-4 steel filaments, a middle sheath layer and an outer sheath layer, in which a space between the steel filaments in each of the middle and outer sheath layers is specified to a given value. In the example of this publication, there is disclosed, for instance, a steel cord of 2+7+12x0.22 structure in which a space ratio between the filaments in the middle sheath layer is 22.6% and a space ratio between the filaments in the outer sheath layer is 21.0%. Furthermore, there are described twisting pitch and twisting direction such that the core is S lay at a twisting pitch of 5 mm, the middle sheath layer is S lay at a twisting pitch of 10 mm, and the outer sheath layer is Z lay at a twisting pitch of 15 mm. In this steel cord, however, the diameter of the steel filament constituting the core is the same as in the sheath, so that the section of the steel cord becomes too elliptical. Therefore, when such a steel cord is used in a cross belt layer of a pneumatic radial tire, there is caused a problem that torsional deformation is apt to be created in the steel cord during the running of the tire to bring about fatigue breakage. In JP-U-64-30398 there is disclosed a steel cord of two-layer or three-layer twisting structure having a core formed by aligning 2-3 steel filaments in parallel with each other. In the example of this publication, the number of the steel filaments in the core is 3 or more and no space for penetrating rubber is formed in the center of the core. When such a steel cord is used in the belt layer of a tire, it is anticipated that there is caused a problem of water penetrating from cut portions formed during running on bad roads into the inside of the cord to corrode the cord. Further, since the diameter of the steel filament constituting the core is the same as in the sheath, fatigue breakage is apt to be caused likewise the aforementioned case. In JP-A-4-257382 there is proposed a steel cord in which the steel filaments in the core, middle sheath layer and outer sheath layer are twisted in the same direction, and the twisting pitch is the same in the core and outer sheath layer, and the twisting pitch in the middle sheath layer is shorter than those of the core and outer sheath layer. In the example of this publication, there is proposed, for instance, a steel cord of 2+5+11x0.22 structure in which the twisting pitch (twisting direction) is 18 (S)/ 9 (S) /18 (S). Such a steel cord has also a problem of fatigue breakage because the diameter of the steel filament in the core is the same as in the sheath. In order to improve the rubber penetrability, when the number of steel filaments in the sheath is too reduced from the compact structure, the tenacity of the cord lowers to impair the function as a reinforcing member, and also the arrangement of the steel filaments in the sheath is disturbed to make the space between the adjoining steel filaments non-uniform and hence the penetration of rubber is rather limited. It is, therefore, an object of the present invention to provide steel cords for the reinforcement of rubber articles having a three-layer twisting structure capable of improving the corrosion resistance and productivity while maintaining the advantages of the multi-layer twisting steel cord of large cord tenacity and good fatigue resistance, and to also provide a method of producing the same. In a first aspect, the present invention provides a steel cord for the reinforcement of rubber articles having a three-layer twisting structure and comprising a core of 2 steel filaments, a middle sheath layer of 6 steel filaments wound around the core, and an outer sheath layer of 11 steel filaments wound around the middle sheath layer, wherein a ratio ds/dc of diameter ds of a steel filament constituting the middle and outer sheath layers to diameter dc of a steel filament constituting the core is within a range of 1.15-1.5, and wherein a twisting pitch of the core is not less than 20 mm. Preferred embodiments of the invention are as follows: the twisting pitch of the core is infinity and the steel filaments in the core are aligned in parallel with each other; the twisting pitch of the core is within a range of 20-50 mm; the diameter of the steel filament in the sheath layers is within a range of 0.20-0.30 mm; the steel filament used in the core and sheath layers is obtained by drawing a steel wire having a carbon content of 0.80-0.85% by weight and has a tensile strength of not less than 3140 MPa; a wrapping wire is wound around the outer sheath layer. In a second aspect, the present invention provides a method of producing steel cords for the reinforcement of rubber articles as mentioned above, which comprises using 2 steel filaments as a core, winding 6 steel filaments as a middle sheath around the core, rendering a helical formed shape of the 6 steel filaments as the middle sheath into a flat shape along the outer periphery of the core of 2 steel filaments through rolls, and then winding 11 steel filaments as an outer sheath around the middle sheath. In a preferred embodiment of the invention, the above steel cords for the reinforcement of rubber articles are used in a cross belt layer of a pneumatic radial tire. The invention will be further described with reference to the accompanying drawings, wherein: Fig. 1 is a schematically sectional view of a first embodiment of steel cord for the reinforcement of rubber articles according to the invention; Fig. 2 is a schematically sectional view of a second embodiment of steel cord for the reinforcement of rubber articles according to the invention; Fig. 3 is a schematically sectional view of conventional steel cord for the reinforcement of rubber articles having a 2+8+14+1 compact twisting structure; Fig. 4 is a schematically sectional view of conventional steel cord for the reinforcement of rubber articles having a 1x3+9+15+1 twisting structure; Fig. 5 is a schematically sectional view of conventional steel cord for the reinforcement of rubber articles having a 1×3+8+13 twisting structure; Fig. 6 is a diagrammatic view showing a part of a step for the production of the steel cord for the reinforcement of rubber articles according to the invention; and Fig. 7 is a diagrammatic view showing a part of another step for the production of the steel cord for the reinforcement of rubber articles according to the invention. In the steel cord for the reinforcement of rubber articles according to the invention, the twisting structure is three-layer twisting structure for ensuring the desired cord tenacity and fatigue resistance. In case of a two-layer twisting structure, it is necessary to increase the diameter of the steel filament for maintaining the given cord tenacity and hence the fatigue resistance is degraded. On the other hand, in case of a four-layer twisting structure, the diameter of the cord becomes thick unless the diameter of the steel filament is made thin and hence the thickness of the rubberized cord layer becomes too thick and the weight of the resulting rubber article undesirably increases. Furthermore, when the diameter of the steel filament is made thin, the productivity in the drawing is undesirably obstructed and also the productivity in the twisting is decreased. The reason why the number of the steel filaments used in the core is limited to 2 is because when the cord contains only a single steel filament as the core, even if rubber penetrates into the core of the cord, the steel filaments of the sheath are eccentrically arranged to helically form a portion not covered with rubber and hence the corrosion resistance is degraded as compared with the core comprised of two steel filaments, while when the core is comprised of 3 or more steel filaments, a space containing no rubber is formed in the inside of the core to degrade the corrosion resistance. Furthermore, the reason why the number of the steel filaments constituting the middle and outer sheath layers are limited to 6 and 11, respectively, is because it is required to maintain a space between adjoining steel filaments sufficient to allow rubber to uniformly penetrate into the inside of the cord. The reason why the ratio of diameter of the steel filament in the sheath to diameter of the steel filament in the core (ds/dc) is limited to a range of 1.15-1.5 is because when the diameter ratio is less than 1.15, the degree of elliptical shape at the section of the cord becomes extreme to easily cause fatigue breakage and cause deviation of the steel filaments in the sheath and hence a wide portion and a narrow portion in the space between the adjoining steel filaments are caused to make uniform penetration of rubber into the inside of the cord impossible, while when the diameter ratio exceeds 1.5, the space between the adjoining steel filaments becomes narrow and it is difficult to penetrate rubber into the inside of the cord. The reason why the twisting pitch of the core is limited to not less than 20 mm is because when it is less than 20 mm, it is difficult to penetrate rubber between the steel filaments in the core and the productivity in the twisting lowers. Moreover, when the twisting pitch is infinity, the rubber penetrability can be improved and the productivity can be considerably enhanced. Even when either or both of the two steel filaments in the core is rendered into wavy or helical form or a combination thereof, the effect aimed at by the invention is not affected. However, when the twisting pitch of the core is more than 50 mm, the fatigue resistance somewhat lowers, so that the twisting pitch is preferably within a range of 20-50 mm. In the steel cord for the reinforcement of rubber articles, the reason why the diameter of the steel filament constituting the sheath layer is preferably limited to a range of 0.20-0.30 mm is because when the filament diameter is not less than 0.20 mm, if the steel cord is applied to a cross belt layer of a tire, sufficient strength of the tire is obtained and excellent resistance to bursting on severe bad roads is developed, while when the filament diameter is not more than 0.30 mm, the fatigue resistance is excellent and the strength per unit sectional area is high and it is possible to reduce the tire weight. Furthermore, the tensile strength of the steel filament used in the steel cord according to the invention is preferably not less than 3140 MPa. According to the invention, the space between the adjoining steel filaments is existent in each of the sheath layers, so that reduced cord tenacity is caused, but if it is intended to maintain the desired cord tenacity by thickening the diameter of the steel filament, the thickness of coating rubber embedding the cords therein undesirably increases to degrade the durability of the rubber article. Therefore, the tensile strength of the steel filament is maintained at a given value instead of thickening the filament diameter. In the production of the steel cord for the reinforcement of rubber articles according to the invention, 6 steel filaments as a middle sheath layer are wound around the core of 2 steel filaments and helically formed and rendered into a flat shape through rolls along the outer periphery of the core. Thus, the steel filaments in the middle sheath layer are arranged without disturbance, so that the space between the adjoining steel filaments in the outer sheath layer is uniformly maintained and hence rubber can easily penetrate into the inside of the cord. That is, it is important that the helical shape of the steel filament in the middle sheath layer is rendered from a circle into a flat shape by disentangling the twisted cord viewing from a direction perpendicular to the longitudinal direction of the cord. In this case, it is preferable that a ratio of major axis C 1 to minor axis C 2 is not more than 0.92. In the pneumatic radial tire according to the invention, when the above steel cords for the reinforcement of rubber articles are used in a tire cross belt layer, it is preferable that a distance between the cords is within a range of 0.70-1.10 mm and that an interlaminar gauge between belt layers is within a range of 0.75-1.15 mm. When the distance between the cords is limited to a range of 0.70-1.10 mm, the resistance to belt separation becomes optimum, while when the interlaminar gauge is limited to a range of 0.75-1.15 mm, the durability in heat generation and the resistance to belt separation become optimum. The following examples are given in illustration of the invention and are not intended as limitations thereof. In the cord of Example 1, a wire material of plain carbon steel having a carbon content of 0.82% by weight is used to form a steel filament for the core having a diameter of 0.185 mm and a tensile strength of 3590 MPa and a steel filament for the sheath having a diameter of 0.260 mm and a tensile strength of 3445 MPa. These steel filaments are twisted under conditions shown in Table 2 to form steel cords. The other production conditions of the steel cord are shown in Tables 1 and 2. The particular twisting method is described with reference to Fig. 6. A take-up reel 5 winding two twisted steel filaments for the core is arranged at the outside of a tubular twisting machine 6, and six reels 7 each winding a steel filament for the middle sheath layer are disposed in the twisting machine 6. The twisted steel filaments for the core are passed through a twisting die 9, while 6 steel filaments are shaped into a helical form by a forming device 8 without substantially twisting and wound around the core in the twisting die. Then, the helical forms of the 6 steel filaments are made flat along the periphery of the core through a plurality of zigzag arranged rolls 10 and wound around a take-up reel 11. Then, 11 steel filaments for the outer sheath layer each having a diameter of 0.260 mm and a tensile strength of 3415 MPa are wound around the two-layer twisted cord taken out from the reel 11 in a twisting machine (not shown) to obtain steel cords of three-layer twisting structure for Example 1. In the production of the steel cord of Example 3, two take-up reels 5 each winding a single steel filament for the core are disposed outside the tubular twisting machine 6 and passed through the twisting die 9 without twisting as shown in Fig. 7. The sections of the steel cords obtained in the examples are shown in Figs. 1 and 2, while the sections of the steel cords in Conventional Examples 1 and 2 are shown in Figs. 4 and 5, respectively. In these figures, numeral 1 is a core, numeral 2 a middle sheath layer, numeral 3 an outer sheath layer,and numeral 4 a wrapping wire. Various pneumatic radial tires having a tire size of 10.00 R20 are manufactured by using various steel cords produced as mentioned above in a cross belt layer of the tire and then the following properties are evaluated as follows. Rubber penetrability into inside of cord After the steel cord is taken out from the cross belt layer of the test tire, the steel filaments for the outer sheath layer are removed to measure a quantity of rubber adhered onto the middle sheath layer, and then the steel filaments for the middle sheath layer are removed to measure a quantity of rubber adhered onto the core. Each of the rubber quantities is evaluated by percentage on the basis that a state of adhering no rubber to the surface is 0%. Adhesion property to saline solution A specimen is prepared by cutting the rubberized steel cord taken out from the cross belt layer of the test tire in a direction perpendicular to the longitudinal direction of the cord so as to expose an end portion of the cord and then immersed in a 10% saline solution for a given time. Thereafter, lengths of rubber portion adhered from the exposed cut end of the cord to the cord surface, middle sheath surface and core surface are measured to evaluate the adhesion property to saline solution. The adhesion property is represented by an index value on the basis that Conventional Example 1 is 100. The larger the index value, the better the adhesion property to saline solution and hence the better the rubber penetrability. Resistance to corrosion fatigue The cross belt layer is taken out from the test tire and immersed in a 10% saline solution for 1 hour after rubber is removed from the cross belt layer at a given place and is subjected to repetitive bending at a radius of curvature of 120 mm under a given loading at a temperature of 50°C and a relative humidity of 90% for a given time to measure the number of broken steel filaments. The resistance to corrosion fatigue is represented by an index value on the basis that Conventional Example 1 is 100. The larger the index value, the larger the resistance to corrosion fatigue. Resistance to separation failure The test tire is mounted on a vehicle and actually run on a bad road until the tire is completely worn. Thereafter, the tire is dissected to measure the presence or absence of separation failure resulting from corrosion of the steel cord through propagation of water into the inside of the cord. The obtained results are shown in Tables 1 and 2. As mentioned above, according to the invention, there can be provided steel cords for the reinforcement of rubber articles which largely improve the corrosion resistance and productivity and have excellent flexibility while maintaining the large cord breaking load and good fatigue resistance inherent to the multi-layer twisting structure cord. Furthermore, when such steel cords are applied to the cross belt layer of a pneumatic radial tire, the rubber penetrability into the inside of the cord is excellent, so that excellent durability and retreading property can be developed.

Claims: A steel cord for the reinforcement of rubber articles having a three-layer twisting structure and comprising a core (1) of 2 steel filaments, a middle sheath layer (2) of 6 steel filaments wound around the core, and an outer sheath layer (3) of 11 steel filaments wound around the middle sheath layer, wherein a ratio ds/dc of diameter ds of a steel filament constituting the middle and outer sheath layers to diameter dc of a steel filament constituting the core is within a range of 1.15-1.5, and wherein a twisting pitch of the core (1) is not less than 20 mm. A steel cord as claimed in claim 1, characterized in that the twisting pitch of the core (1) is infinity and the steel filaments in the core are aligned in parallel with each other. A steel cord as claimed in claim 1, characterized in that the twisting pitch of the core (1) is within a range of 20-50 mm. A steel cord as claimed in any of claims 1 to 3, characterized in that the diameter of the steel filament in the sheath layers is within a range of 0.20-0.30 mm. A steel cord as claimed in any of claims 1 to 4, characterized in that the steel filament used in the core and sheath layers is obtained by drawing a steel wire having a carbon content of 0.80-0.85% by weight and has a tensile strength of not less than 3140 MPa. A steel cord as claimed in any of claims 1 to 5, characterized in that a wrapping wire (4) is wound around the outer sheath layer (3). A method of producing steel cords for the reinforcement of rubber articles as claimed in claim 1, comprising using 2 steel filaments as a core (1), winding 6 steel filaments as a middle sheath (2) around the core, rendering a helical formed shape of the 6 steel filaments as the middle sheath into a flat shape along the outer periphery of the core (1) of 2 steel filaments through rolls, and then winding 11 steel filaments as an outer sheath (3) around the middle sheath (2). A pneumatic radial tire having a cross belt layer, characterized in that steel cords as claimed in any of claims 1 to 6 are used in said cross belt layer. A pneumatic radial tire as claimed in claim 8, characterized in that a distance between the steel cords is within a range of 0.70-1.10 mm, and in that an interlaminar gauge between belt layers is within a range of 0.75-1.15 mm.

Inventors: KUDO EIJI, YAMANAKA TAKAYA, KUDO, EIJI, YAMANAKA, TAKAYA, KUDO, EIJI, C/O BRIDGESTONE METALPHA CORPORATION
Applicants: BRIDGESTONE CORP, BRIDGESTONE METALPHA CORP, BRIDGESTONE CORPORATION, BRIDGESTONE METALPHA CORPORATION
Application year: 1994
Main classification: D07B1
IPC codes: D07B1, B60C9